The invention relates to a method of converting a digitally modulated reception signal from the radio-frequency range, in which the reception signal, which can be represented as a complex vector, is converted in-coherently into the baseband. The invention relates furthermore to a receiver for digitally modulated signals in a mobile communication system, the antenna of which is connected to at least one mixing stage for the reception signal which can be represented as a complex vector, the mixing stage being followed by two low-pass filters which are connected on the output side via an analog-digital converter in each case to a value converter.
A receiver operating by such a method is known from PCT International Publication WO 86/3356. The reception signal is converted by two mixers with a local oscillator in correct phase relation and phase-shifted by 90.degree.. Low-pass filters and analog-digital converters are followed by a digital filter. The decoding of the transmitted bits is performed with a ROM, operating as a value converter, which is followed by a multiplexer (FIG. 17).
A reception method and a receiver for cordless telephones is described in Telcom Report 10 (1987), issue 2, pages 130 to 137. The reception signal is mixed into the baseband via a radio-frequency stage and a plurality of intermediate-frequency stages. The demodulation is performed in phase-locked loop.
In general, digitally modulated signals are processed by synchronous or, more rarely, asynchronous heterodyne receivers. An image frequency suppression is performed at the radio-frequency stage, the main selection at one to two intermediate-frequency stages. Limiter amplifiers or gain controls provide the necessary compensation of the dynamic range of the input signal. Linear modulation methods divide into two-stage methods with binary amplitude modulation (BAM) and binary phase-shift keying (BPSK), four-stage methods with quadrature amplitude modulation (QAM) and quadrature phase-shift keying (QPSK), as well as multistage modifications. Customary non-linear modulation methods are FSK or frequency modulation-like methods, such as minimum-shift keying (MSK) and Gaussian minimum-shift keying (GMSK). The demodulation is performed for non-linear modulation methods by means of phase lock loop, Costa's loop or discriminators. For linear modulation methods, synchronous quadrature demodulators are used.